In the last several years, minimally invasive surgical procedures, such as percutaneous transluminal coronary angioplasty (PTCA), have become increasingly common. A PTCA procedure involves the insertion of a catheter into a coronary artery to position an angioplasty balloon at the site of a stenotic lesion that is at least partially blocking the coronary artery. The balloon is then inflated to compress the stenosis and to widen the lumen to allow an efficient flow of blood through the coronary artery.
Following PTCA and other stenotic treatment procedures, a significant number of patients experience restenosis or other vascular blockage problems. These problems are prone to arise at the site of the former stenosis.
In order to help avoid restenosis and other similar problems, a stent may be implanted into the vessel at the site of the former stenosis with a stent delivery catheter. Stents are described in U.S. Pat. No. 4,733,665 to Palmaz, U.S. Pat. No. 4,800,882 to Gianturco, and U.S. Pat. No. 4,886,062 to Wiktor.
FIG. 1 illustrates a conventional stent 10 formed from radially expandable struts 12 that are interconnected by connecting elements 14. Lateral openings or gaps 16 are formed between adjacent struts 12. The struts 12 and the connecting elements 14 define a tubular stent body having an outer, tissue-contacting (abluminal) surface and an inner (luminal) surface.
Stents can also be used to deliver drugs locally. Local delivery is often preferred over systemic delivery, particularly where high systemic doses are necessary to affect a particular site. High systemic doses of drugs often create adverse effects. For example, following angioplasty, radiotherapy and drug delivery treatments applied to the former stenosis have been found to aid in the healing process and to reduce significantly the risk of restenosis and other similar problems. One proposed method of local delivery is to coat a stent surface with one or more drugs.
There are several conventional methods for coating a stent with a drug, e.g. by dipping the stent in a coating substance containing the drug or by spraying the solution onto the stent. Dipping and spraying usually results in completely coating all stent surfaces, i.e., both luminal and abluminal surfaces. While the coating on the abluminal surface provides an advantageous direct delivery of the drug to the site of the former stenosis, the coating on the luminal surface can be washed away by the blood, which in some cases makes it therapeutically insignificant.
Moreover, the luminal surface coating often detrimentally affects stent deliverability and the coating's mechanical integrity. The luminal coating may increase the friction coefficient of the stent's surface, making withdrawal of a deflated balloon more difficult. The coating may also adhere to the balloon. Consequently, balloon deflation and withdrawal may damage the coating or remove portions of the coating from the stent, resulting in a thrombogenic stent surface and embolic debris.
The dipping and spraying methods have additional shortcomings. For example, these methods tend to cause webbing between adjacent stent struts and coating pools on the stent, making it difficult to control the amount of drug coated on the stent. Additionally, the spraying method may cause coating defects at the stent-stent-mandrel interface. Upon removal from the stent mandrel, the coating material at the interface may detach from the stent, leaving uncoated stent areas.
To overcome the above shortcomings, piezoelectric delivery systems have been developed, which deliver coating droplets to specific stent surfaces, allowing a more precise coating of the stent. However, these systems also have several drawbacks. For example, they do not consider several factors that affect droplet size (or volume). For example, the droplet size is affected by the coating substance's viscosity or density. The higher the viscosity or density, the smaller the droplet size. In addition, nozzle clogging also affects droplet size. As a result, these conventional piezoelectric delivery systems cannot precisely control the delivery rate of coating substance.